The rate of growth of voice traffic on computer data communication networks has been phenomenal since the last decade of the last century. In contrast to conventional voice communication systems in which voice is carried as a stream of synchronous data over PSTN networks by circuit switching using time-division multiplexing (TDM), computer networks typically carry voice traffic primarily as data packets over packet switched data networks. This form of voice traffic is commonly referred to as packetized voice traffic because packets of voice data form the basis of communication. Because of the various advantages of using private internal networks, such as LANs, to carry packetized voice traffic in an enterprise environment, as compared to the use of conventional stand-alone and dedicated PABX systems for internal voice traffic switching, internal networks carrying packetized voice traffic are increasingly used by enterprises to replace dedicated PABX systems. Examples of such advantages include more convenient and efficient system management since a single network infrastructure can be shared by both voice and data traffic and enhanced scalability.
An important application of packetised voice data traffic is the carrying of voice over the Internet. The Voice Over Internet Protocol (VoIP) is widely accepted as the industrial standard protocol for such purposes. Nowadays, VoIP as a transmission protocol has found wide applications in both internet and non-internet applications. For example, VoIP is also used in packetized voice data communication applications in private communication networks. Hence, the term VoIP and the standard protocol itself is no longer restricted to voice communication over the Internet and the description below should be understood on that basis.
VoIP, especially VoIP using Session Initiation Protocol (SIP), is becoming increasingly important for enterprises phone applications because quality voice traffic can be provided at lower costs, with enhanced flexibility and controllability. However, security and reliability remain the major concerns and these might have prevented a large-scale deployment of IP telephony in commercial or enterprise environment thus far.
Typically, an IP telephony network is built on top of or embedded in the enterprise data network or LAN. This may be a result of maximising the utilisation of existing computer networks, or a preference for centralised and enhanced management of data and voice traffic within a corporate environment or other practical reasons. However, this conventional setup means outage of the data network, for example, due to hacking, will also result in the outage of the corporate telephone system which is clearly not acceptable.
Hence, it will be desirable if shortcomings of conventional VoIP networks can be alleviated so that a compromised data network (LAN or VLAN) will not adversely affect the IP-based voice network of an organisation which is physically connected with the data network. Consequently, a secure and reliable IP-based voice network can be deployed to take advantage of the Internet or other network or network protocols as and when available.